By 2517, then-Chief Petty Officer Mendez was regarded as the Navy's best hand-to-hand combat and physical training drill instructor, in addition to having substantial combat experience. Due to his skills, he was recruited to lead the training of the SPARTAN-II supersoldiers.[4] He was the SPARTAN-IIs' training leader during their first years on Reach from 2517 until 2525. He trained them in the aspects of combat as hard as any Marine when they were only at the age of six.[5] His military training was punctuated with in-class strategy sessions with AI Déjà.[6]

At the beginning of their training, CPO Mendez and his training instructors used electric batons to discipline trainees.[7] However, his trainers would occasionally use rifles with tranquilizer rounds to incapacitate the trainees (those who would eventually make up Gray Team) who attempted to escape. Eventually the Spartans, John-117 in particular, came to revere Mendez as a solid, unmovable force in their lives.[8] They had great respect for him, perhaps even to the extent of seeing him as a father figure, much as they regarded Dr. Catherine Halsey as a mother figure.

After the SPARTAN-IIs' training was completed, Mendez was reassigned to train the next class of Spartans.[9] John-117 never saw him again, though Mendez returned the quarter that Dr. Halsey had given John during their first meeting. He recalled that John broke a few of the trainers' fingers when they tried to take the coin from him.[10]

While training the final batches of Gamma Company on October 31, 2552, Mendez and the other UNSC forces on Onyx came under attack by ForerunnerSentinels. He, along with SPARTAN-III Team Saber, Tom-B292, Lucy-B091, and Ambrose were virtually alone on the planet, and were forced to hide from Sentinels for several days. Eventually, SPARTAN-II Kelly-087 and Dr. Halsey arrived, followed shortly by SPARTAN-II Blue Team: Fred-104, Will-043, and Linda-058. Mendez assisted them in fighting their way to the core of Onyx, where he eventually evacuated to Shield World 006, held in a slipspace bubble in Onyx's core, along with the other survivors. Onyx, actually an artificial planetary shell covering the shield world, was then disintegrated by the Sentinels.[11]

At the funeral for the Spartans who had died on Onyx, Mendez read an excerpt from his favorite book: an account of the Rain Forest Wars called A Soldier's Tale: Rainforest Wars.[12] While inside the shield world, Mendez began to reflect upon his role in training the Spartans.[13] He also took note of Halsey's discomfort with the SPARTAN-IIIs he had helped train, confronting the doctor about this on several occasions.[14]

Mendez is a strict, by-the-book military man and thus has a solid sense of duty and honor. He also rarely allows himself to display his emotions (for example, John-117 very rarely had seen him smile). When he heard of the destruction of the Spartan-IIIs, he kept his face stony, holding his feelings to himself. His total need for discipline and order off-sets many of his new acquaintances, however those who know him well (namely the Spartan-IIs, particularly John-117 and Kurt Ambrose) have a respect and relationship with him that can be compared to that of the connection between father and child.

Mendez's stringent sense of military discipline extends to himself, and he is known to devote considerable attention to his physical fitness even with age. When he arrived to train Alpha Company, Lieutenant Commander Ambrose noted that Mendez had aged very well, that he had a trim chiseled body from "ironwood", with hair that was now silver, deep creases around his eyes and that a set of ragged scars that ran from brow to chin.[2] By 2552, Kurt still found Mendez him to be surprisingly fit and agile, noting he moved like a soldier thirty years younger and wondered what kind of Spartan he would have made.[18]

^In Halo: The Fall of Reach, Mendez is described as a veteran instructor who had also seen significant combat by 2517. In Halo: Ghosts of Onyx, Kurt-051 notes that Mendez had "aged well" in 2531. Later in Ghosts of Onyx, however, Mendez is noted as "pushing sixty" in 2552, and is stated to be the same age as Dr. Halsey in Halo: Glasslands. This would make him no more than 25 years old when he started training the SPARTAN-IIs and around 39 when Kurt mentions his aging. Furthermore, the significant spans of cryosleep which accompany UNSC military service would likely reduce his biological age even more, making Kurt's remark seem even more out of place.